U.S. Pat. No. 7,112,256 B2 reveals a method for improving the heat economy in a continuous digestion system. The digestion system comprises an impregnation vessel and a digester. Chips are fed into the impregnation vessel and are there impregnated at a pre-determined impregnation temperature, before the impregnated chips are fed out from the impregnation vessel up to the top of the digester in order to be cooked. A dissolved cellulose pulp is fed out from the outlet at the bottom of the digester when the cooking process is complete. At least one black liquor withdrawal is taken from the digester, and this is added to the lower part of the impregnation vessel, in order to increase in this way the temperature of the chips before they are further fed upwards to the top of the digester vessel. The need to add hot steam at the top of the digester is in this way reduced. A part of the black liquor is then withdrawn from the top separator of the digester in order to be added in the impregnation vessel and there used as impregnation fluid. This invention allows the achievement of a “cold” black liquor impregnation, where the black liquor before it is taken to the impregnation vessel has to a large extent cooled by convection in the lines and through its mixing with the colder impregnation fluid and the chips. The requirement for the addition of steam at the top of the digester has at the same time been considerably reduced. FIG. 2 shows an alternative embodiment in which a cooler (21) is used to reduce even further the temperature of the black liquor that has been withdrawn from the top separator.
The use of a heat exchanger, known as a “reboiler”, that generates steam in cooking processes is known. U.S. Pat. No. 6,176,971 B1 reveals a cooking process in which hot black liquor, directly withdrawn from a digester vessel (and possibly having been cooled in a heat exchanger), is used to create pure steam with the aid of a reboiler (71). The pure steam (73) is then led to a steam pre-treatment vessel (17) in order there to heat the chips. The black liquor is sent after its passage through the reboiler to a recovery process.
A cooking process is shown also in U.S. Pat. No. 6,306,252, FIG. 2, where the production of pure steam from water takes place. The black liquor in this case is withdrawn directly from the digester and it heats the water in a heat exchanger (19) before the heated water is led to flash tanks (21, 22) for the production of pure steam. The black liquor is led after its passage through the heat exchanger (19) to a recovery process.
FIG. 3 shows an alternative embodiment in which pure water is led to a pre-vaporiser (27). The water exchanges heat in the pre-steamer with hot black liquor (11), where the water is vaporised to pure steam. The pure steam can then be used in order to treat chips with steam during impregnation, while the cooled black liquor can be sent to a recovery process, can pre-treat chips in the digester, can be used as a source in order to create more pure steam, or can be flashed off in order to produce impure steam.
A first object of the present invention is to produce pure steam in order to pre-treat the chips before the chips are to be impregnated in an impregnation vessel.
A second object is to exploit a withdrawal of digestion fluid from the top of the digester with the aim of obtaining the pure steam.
A third object is to produce the pure steam by using indirect exchange of heat between the withdrawal from the top of the digester and a pure fluid, where the pure fluid is converted to pure steam.
A fourth object is to obtain by indirect heat exchange a cooling of the cooking fluid withdrawn from the top of the digester, where the cooled withdrawal is subsequently used as impregnation fluid.
A fifth object is to exploit the heat energy in a more efficient manner.
A sixth object is to obtain an impregnation process in which the impregnation fluid that is led to the impregnation vessel has a low temperature.
The invention concerns a method for the production of cellulose pulp in a continuous digester system in an energy-efficient manner. The method comprises an impregnation vessel in which to impregnate the chips, which chips are then fed to a subsequent digester vessel in a transfer fluid. A black liquor withdrawal is taken from the digester, which withdrawal is led to the bottom in order there to heat the chips before they are fed out from the impregnation vessel. A withdrawal of the transfer fluid is taken from the top of the digester and led to a position in order there to act as impregnation fluid in the impregnation vessel. The invention is characterised in that at least a portion of the transfer fluid that is withdrawn from the top of the digester passes an indirect heat exchanger, in which the transfer fluid withdrawn from the top of the digester at a temperature of at least 125° C. exchanges heat indirectly with a first fluid for the production of steam from the first fluid. The steam that is produced is then led to a steam pre-treatment position, upstream of the impregnation process, in order to heat the chips at the said steam pre-treatment position.
The following positive advantages over the prior art technology are obtained with the use of the invention:                The impregnation fluid that is withdrawn from the digester and led to the impregnation vessel is cooled in association with the production of the steam. This is advantageous for the impregnation.        The conversion of pure fluid to pure steam takes place in a manner that is highly energy-efficient.        The heat energy that leaves the impregnation fluid is absorbed by the fluid that is converted to steam, while the impregnation fluid at the same time is cooled. This ensures that not only a cold impregnation but also a pre-heating of the chips by steam before impregnation are obtained in a very energy-efficient manner.        